Oh Cruel EMT, Why Make A liar Out Of Me.....?
I just installed another 1 GB of memory in my other computer. This brings the total to 3 GB. Wonder of wonders, Windows now reports this as 2.98 GB. This is about full recognition since BIOS commandeers some memory. There are qualifiers: Only 1 USB device is connected( a joystick), This is an G965WM Intel board with EMT, The CPU is a Celeron D 356 (it's a Cedar Mill 64 bit). The board in question states that it supports 8 GB of memory (Well, not the board, obviously, the manual). I think (think may be overstated), that by adding a 3rd abstraction layer it is possible to support more than 4 GB of memory with a 32 bit OS. <(Starting with "I think", that's speculation).
As to if (having committed to installing a 64 bit OS), it should be XP or Vista,
consider this, I believe that MS has fixed date certain for withdrawal of support for XP. This would mean no more security updates or tech support. So, you would be growing into Vista but growing out of XP. In either case I would clean the HDD, and install a full version (not an upgrade) of the OS. As always, get a second opinion on that.
But for free we have, Plan C: Download an install a 64 bit version of Linux. Ubuntu Linux:
http://www.ubuntu.com/ can be run from directly from the CD drive without installing (I can only speak with certainty on the 32 bit version). Trying something on before you buy it always seems a sensible course of action.
Trivia: Burning a DVD with Nero, memory usage tops out at about 750 MB with 2 GB RAM installed. With 3 GB of RAM installed, burning a DVD with Nero, memory usage tops out at about 750 MB. This with TV 1 hour episodes, I'll try something feature length later.
The issue with Vista, up until now, has been driver support. This applies especially to graphics drivers. So, Vista & 64 bits together could possibly double the aggravation. With respect to your particular cards, it might serve you well to post that question in the Audio-Video Forum.